A word about scores
When a piece of music is to be performed repeatedly the same way, or performed by people who weren't the originators of the piece, it helps to have a score. In the history of objectifying music, scores were the currency until they were displaced by recordings. There are many types of microtonal notation, such as Sagittal notation, color notation and ups and downs notation.

Historically significant microtonal scores | Links to Microtonal Scores, by Composer

Historically significant microtonal scores

Check your local gigantic university library. If they don't have it, they could probably be persuaded to get it.

Charles Ives's Three Quarter Tone Pieces (1913-1924)
Harry Partch
Ben Johnston, esp. string quartets
Alois Hába, esp. string quartets
Easley Blackwood, esp. Twelve Microtonal Etudes for Electronic Music Media, op. 28
publishings of Diapason Press
this list is so obviously incomplete that you should help finish it.

Links to Microtonal Scores, by Composer

Please contribute to this list of scores that are available (i.e. downloadable). This is meant as a resource for performers interested in microtonal music. Feel free to make wiki pages for "notable" compositions; the wiki supports file attachments up to 10 MB each.

Jacob Barton

De-quinin' for two Bb clarinets (one tuned 33¢ flat)
Moods for 3 keyboards
Kluh for 11 performers on 21 tuned bottles
Eighty-one ninth chords for 2 pianos, 17edo
Land Urchin for string band in 11-limit just intonation

David Canright

Fibonacci Suite (piano retuned to 7-limit JI, seven hands)

William Copper

Ivor Darreg

On the Enharmonic Tetrachord (from Suite, Op. 62), in 22tET. Originally printed in the Spring 1975 issue of Xenharmonikon in quarter-tone notation. Transcribed to Sagittal by Juhani Nuorvala.

Paul Erlich

Tibia in 22tET (Listen). Sagittal score in F||\ or in G

Kyle Gann

The Day Revisited for flute, clarinet, electric bass, 2 keyboards
Charing Cross for keyboard and tape
New Aunts for keyboard
Triskaidekaphonia for keyboard
Fugitive Objects for keyboard
Custer and Sitting Bull, "electronic cantata"
Love Scene for string quartet. Gann creates tuning guides for many of his pieces, list at the bottom of this page.

Robert Hasegawa

Spiral for chamber ensemble
Chaconne for James Tenney for chamber ensemble (JI/96edo
Due Corde for 2 pianos, 19edo

Andrew Heathwaite

Clouds, 72edo
Prayer of Thanks, 7-limit just intonation

Paul Kotheimer

Goes to Eleven (2009) for ensemble, 11-limit JI
Three Short Poems (2008) for voices and drones, 5- and 7-limit JI
Mercy (2008) for voice and drone (and udderbot), 13-limit JI

Mats Öljare

Sunday Pipes, 22edo

Tui St. George Tucker

three Quartertone Lullabies for wind trios
Vigil I and Vigil II for organ
Little pieces for quartertone piano
Sarabande for quartertone harpsichord

Dante Rosati

"Arbor Low" for 21-tone Just Intonation guitar

Thomas Edwin Scheurich III

Three Lonely, Uninspired Ways to Play in 17tet (2006) for 2 pianos
Attention! Attention! Attention! (2007) for 17-tone ensemble (soprano, udderbot, slide whistles, violin, viola, cello, pianos, laptop, percussion)
Story Worth Telling (2008) chamber operetta in 17edo
Xenharmonic Curiosities II (2009) for voices and viola